This invention relates generally to the field of membrane study, biosensor/biomembrane development and manufacture, and nano-scale photovoltaics. The invention relates more particularly to a method and apparatus for photoelectrically applying a voltage across a membrane for membrane channel addressing, power supply and switching for nanodevices, nano-scale signal transmission and other applications.
The application of a voltage across a biological or synthetic membrane can be utilized in a variety of applications. For example, application of a localized voltage across a membrane can be used to address or map biological structures such as ion channels and/or to detect binding events at a channel. An amphotericin B ion channel within a supporting membrane, for example, is held open by a cholesterol molecule that zips the amphotericin molecules together in a channel-forming configuration, allowing ion transport through the membrane. When a free analyte binds to the associated antibodies of the amphotericin molecules, the cholesterol molecule is displaced and the channel is unzipped, resulting in disaggregation and closing of the channel, thereby blocking ion transport across the membrane.
The presence or absence of such ion channels or carriers in a membrane can act as a molecular switching element that converts a binding event into an electrical signal, functioning as a transducer in a biosensor or nanodevice. For example, in a membrane in which a molecular channel or switch is held open when a specific analyte is bound, ion transport through the membrane is permitted when the analyte is bound, but is blocked when the analyte is not bound. If a voltage is applied across the membrane, a current pulse will be observed if ion transport occurs through the membrane, indicating an open channel and thus the presence of a binding event. Conversely, if a voltage is applied across the membrane and no current is observed (i.e., no ion transport through the membrane), a closed channel (and thus the absence of a binding event) is indicated.
The very small scale of the membranes and the molecules forming ion channel and ion carrier molecular switches under investigation (commonly on the order of about 100 Angstroms), as well as the relatively high density of ion channels on a substrate renders the addressing of these channels very difficult using known techniques. One conventional solution for the addressing of biological structures such as ion channels would be to make electrical connections to all or to many of these molecular switches. The applied voltage and responses of individual addresses on a substrate such as a silicon wafer surface could be scanned with the aid of computerized circuitry. However, the resolution of known addressable electrodes is poor, and manufacturing of an electrode system on the substrate surface would likely prove difficult and expensive. Also, voltage applied to a membrane in an electrolytic solution is typically conducted through the electrolyte along the membrane surface, rendering it difficult or impossible to address or map a specific location on the membrane.
It is also known to utilize a scanning ion conductance microscope to image the topography of soft non-conducting surfaces covered with electrolytes by maintaining a micropipette probe at a constant conductance distance from the surface. This method can sample and image the local ion currents above the surface by scanning with a micropipette probe in a plane located at a constant distance above the surface. Multiple micropipettes mounted in a multi-barrel head and containing various ion specific electrodes allow simultaneous scanning for different ion currents. The resolution of this method, however, is low and the method is tedious and costly.
Scanning ion conductance microscopy (SICM) techniques for assessing the volume of living cells allow quantitative, high-resolution characterization of the dynamic changes in cell volume while retaining the cell""s functionality. This technique is reportedly capable of measuring a widerange of volumes. The volume of small cellular structures such as lamelopodia, dendrites, processes, or microvilli, can purportedly be measured with 2.5xc3x971020 resolution. The sensitive probe of this method is a glass micropipette filled with electrolyte and connected to a high-impedance head-stage amplifier that is mounted on a computer-controlled three-axis translation stage. This method, however, is also unwieldy and costly.
The application of a voltage across a membrane may also find application in energy delivery, switching and/or signal transmission for nano-scale devices (xe2x80x9cnanodevicesxe2x80x9d), and in other fields of endeavor. To date, however, these areas of technology have not been developed to a significant extent, likely due at least in part to the lack of suitable methods and apparatus for locationally precise voltage application.
Thus, it can be seen that needs exist for improved methods and apparatus for applying a voltage across a membrane. It is to the provision of improved apparatus and methods meeting this and other needs that the present invention is primarily directed.
The present invention provides an improved apparatus and method for applying a voltage across a membrane. Example embodiments of the apparatus and method of the present invention enable precise locational control of the point of application of voltage across a membrane, providing improved addressing of biological structures such as ion channels and improved detection of binding events at a channel. In other embodiments, the method and apparatus of the present invention provide photoelectric power supply, microswitching, energy transmission and/or signal transmission suitable for use in connection with nanodevices including without limitation: nanomotors, nanoswitches, nanotranslators and/or micropositioners.
In one embodiment, the method and apparatus of the present invention uses a photoelectric method to generate a voltage across a membrane applied to a substrate, in which the transmembrane voltage is used to address biological structures such as ion channels or molecular switches on the substrate surface. Application of light on a semiconductor/electrolyte-liquid interface generates an electrical charge gradient due to the photovoltaic effect. The electrochemical gradient drives electric current carriers (electrons and/or ions through the interface. When a membrane with open ion channels is applied to the substrate at the semiconductor/liquid interface, the current induced by the light flows relatively freely through the open channels, resulting in a relatively large induced current. When the channels are closed or blocked, the current through the membrane is small or non-existent.
By scanning the substrate surface with a narrowly-focused laser beam or other light source and monitoring the current induced by the application of light at each location along the scanned path, channel opening events are observed and the electrical topography of the surface can be addressed and recorded. The laser beam illuminates a small portion of the substrate/liquid interface, creating an electrochemical gradient that drives current through any open ion channel (or small group of channels) located at the illuminated position. By sequentially moving or continuously scanning the point of illumination with the laser beam across the surface, the electrical pattern and topography of the substrate is generated and the ion channels are mapped. In example embodiments, this method of photoelectrically addressing biological structures such as ion channel switches offers the advantages of high resolution of the electrical topography of the surfaces, low cost and simplicity.
In one aspect, the invention is a method of generating a voltage across a membrane. The method preferably includes applying a membrane to a semiconductive substrate, and illuminating at least a portion of the semiconductive substrate with a light source.
In another aspect, the invention is a method of observing molecular channel opening events and addressing the electrical topography of a membrane. The method preferably includes the steps of providing a semiconductive substrate having a membrane deposited thereon in contact with an electrolytic solution to form a semiconductor/liquid interface, scanning successive portions of the semiconductive substrate with a light source to generate a localized electrical charge gradient at a location on the semiconductor/liquid interface, and measuring an electrical current through the membrane.
In yet another aspect, the invention is an apparatus for generating a transmembrane voltage. The apparatus preferably includes a semiconductive substrate having a surface for receiving a membrane thereon, and a light source for illuminating at least a portion of the semiconductive substrate.
These and other aspects, features and advantages of the invention will be understood with reference to the drawing figures and detailed description herein, and will be realized by means of the various elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following brief description of the drawings and detailed description of the invention are explanatory of example embodiments of the invention, and are not restrictive of the invention, as claimed.